Recently, a magnetic recording and reproduction apparatus has been increasing recording density to realize small and large capacity. Especially, in the field of a hard disk as a typical magnetic recording device, an areal recording density of more than one gigabit per square inch is already available on the market, and an areal recording density of ten gigabits per square inch is expected in a couple of years. The technology proceeds with a rapid pace.
One of the primary factors that has enabled such high recording density is the increasing linear recording density, due to improvements of medium properties, head-disk interface performance, and new signal processing method such as “partial response”. However, in recent years, the rate of increase in track density exceeds that of linear record density, and thus becomes a primary factor of the increasing areal recording density. Practical use of a magneto-resistive type head, which is superior to a conventional inductive type head in read-back signal performance, has contributed to the progress in the track density. It is possible at present to read a signal from a track whose width is only a few microns with good S/N ratio by practical use of the magneto-resistive type head. On the other hand, it is expected that a track pitch will reach the sub-micron range in the near future along with further improvement of the head performance.
A tracking servo technique is important for the head to read a signal with high S/N ratio by tracing such a narrow track. For example, a conventional hard disk has areas that are located at predetermined angles over 360 degree and in which information such as a tracking servo signal address and clock signal are written. In this specification, preformat writing or prewriting of such an information signal is called a “preformat recording”. A head can trace a track by reading such information in predetermined intervals, and monitoring and correcting the head position.
The above mentioned tracking servo signal, address and clock signal become reference signals for the head to trace a track precisely. Therefore, precise record positions are required for these information signals. Current preformat recording into a hard disk is usually performed by magnetic heads placed in the hard disk drive by using a special servo track writer after installing the disk and the head into the drive. In this case, a required accuracy of the track position for writing is achieved by precisely controlling the position of the head incorporated in the drive by using an external actuator equipped to the servo track writer.
Such a preformat recording of servo signal address information and clock signal is performed similarly for large capacity flexible disks or disk cartridges, which are removable disk media seen in the market recently, by using a magnetic head and a servo writer. These media are removable, so they can be compatibly used by other drives. Therefore, it is not always required to perform the preformat writing by the heads of each drive after incorporating the heads into the drive though it is required for a normal hard disk. However, these removable disks are similar to normal hard disks from the viewpoint that the preformat writing is performed by precisely controlling the position of the head by using an external actuator equipped to the servo track writer.
However, in the present preformat recording of servo signal, address information and clock signal, there are the problems described below.
The first problem is that writing with the magnetic head is a linear recording relying on relative movement between the magnetic head and the recording medium. This means that a long period is required for preformat writing by the above-mentioned method, while precisely controlling the position of the magnetic head with a servo track writer. Moreover, because the servo writer is expensive, the cost for preformat writing is high.
This problem becomes even more serious as the areal recording density increases. This is not only caused by an increase of tracks in radial direction. As the track density increases, a higher precision is required for the head positioning and as a result, servo areas, in which the tracking servo signal and other signals are recorded, have to be provided with smaller angular distances between them over 360 degrees. Moreover, the address information to be written as the preformat data increases as the recording density increases. Thus more time and cost are required for writing more information signals as the record density becomes higher.
A smaller size for magnetic disks is expected to be the trend on the market. However, large disks of 3.5 or 5 inch size are still in demand. These large disks require more information signals to be written for the preformat than the small disks. The necessary time for preformat writing influences the cost effectiveness of such large disks.
The second problem is that a space between the head and a medium or a diffusive recording magnetic field due to a pole shape of the record head does not make a steep magnetic transition at track edges where the preformat data is written. Relative movement between the magnetic head and a medium is indispensable in writing with the head, so some space is necessary between the head and the medium for interface performance between them. A conventional magnetic head usually has two elements for writing and reading. A pole width at a trailing edge of the head corresponds to a record track width, and a pole width at a leading edge is several times larger than that at the trailing edge.
The above two phenomena may be a factor for causing the diffusive recording magnetic field to fringe over the preformatted record track width, resulting in the magnetic transition at track edges not being steep or erased areas appearing on both sides of a track. In current tracking servo techniques, the head position is detected by a change in read signal amplitude when the head misses a track. Therefore, as in the process of reproducing the data signal recorded between the servo tracks, the system requires not only a high S/N ratio of a read signal when the head traces a track correctly, but also a steep off-track performance, in which the read signal amplitude changes steeply as the head misses the track. If the magnetic transition is not steep enough at an edge of a track where the preformat is written, it is difficult to realize a precise tracking servo performance that will be required for a submicron track recording in the future.
As a solution of the first of the two problems mentioned above, a duplicate record technique of a tracking servo signal or other signals by using a magnetic transfer technique has been disclosed in Japanese Publication of Unexamined Patent Application (Tokukai) Sho63-188628. The duplicate record technique of a magnetized pattern using the magnetic transfer technique was originally developed as a method for copying the contents of a videotape. This technique is explained in detail in C. D. Mee and E. D. Daniel, “Magnetic Recording”, Vol. 3, Chapter 2, p94-105, for example. The method disclosed in Tokukai Sho63-183623 applies the above duplication technique for videotape to the preformat writing of the tracking servo signal or other signals for a flexible disk.
Such a magnetic transfer technique may improve the productivity of the preformat writing. However, this technique is effective only for media such as flexible disks that have a small coercive force and a low areal record density. It is not effective for today's hard disks, which have a large coercive force and a high areal record density in the order of several hundred megabits to gigabit.
In the magnetic transfer technique, an alternating bias magnetic field has to be applied, whose amplitude is approximately 1.5 times the coercive force of the target (slave) disk to ensure high transfer efficiency. The coercive force of the master disk should be more than three times of that of the slave disk, so that the master information, i.e. a magnetized pattern in the master disk, is not erased by the alternating bias magnetic field. Today's high-density hard disk media have a coercive force of 120-200 kA/m to enable a high areal recording density. It is estimated that the coercive force will reach 250-350 kA/m for an areal record density of 10-gigabit order in the future. This means that a master disk should have a very large coercive force of 360-600 kA/m at present and 750-1050 kA/m in the future.
It is difficult to realize such a large coercive force for a master disk from the standpoint of a magnetic material. In addition, master information cannot be written into a master disk having such a large coercive force by any current magnetic recording method. Therefore, considering a possible coercive force for a master disk in the current magnetic transfer technique, the coercive force of the slave disk inevitably has an upper limit.
In the above-mentioned magnetic transfer technique, it is possible to utilize a thermo-magnetic transfer technique, where instead of applying the alternating bias magnetic field to the slave disk, the slave disk is heated to the temperature near to the Curie temperature for eliminating spontaneous magnetization. However, in that case, the Curie temperature of the slave disk should be much lower than that of the master disk. High coercive force magnetic film composed of Co group materials used for a high density magnetic record medium has a relatively high Curie temperature, so it is difficult to realize the characteristics required of the master disk and the slave disk for the thermo-magnetic transfer. Therefore, this preformat writing with a magnetic transfer technique cannot be a substantial solution for the before-mentioned problems.
Another solution for these problems is a pre-embossed disk technique disclosed in Publication of Japanese Unexamined Patent Application (Tokukai) Hei7-153060 (corresponding to U.S. Pat. No. 5,585,989 and European laid open patent application No. 655,734). In this technique, an embossed pattern corresponding to a tracking servo signal, address, clock signal and/or other signals is formed on a surface of the disk substrate by a stamper, and a magnetic film is formed on the substrate. This technique can be an effective solution for the before-mentioned problems. However, the embossed pattern on the disk surface may influence the head's flying float performance (or contact state in the case of contact writing) when writing or reading, so that interface performance between the head and medium may be problematic. In addition, the substrate processed by the stamper is usually a polymer material (plastic), so it cannot be heated when forming the magnetic film for ensuring medium properties, and thus a necessary S/N ratio cannot be ensured.
As mentioned above, a truly effective solution of the before-mentioned two problems, which does not sacrifice other important performance such as the medium S/N ratio or the head-medium interface, has not been found yet.